The objective of this project is to study specificities of macrocyclic ligands for cations as a function of (a) cation charge, size, and type; (b) ligand cavity size; (c) type, number and spatial arrangement of both ligand and donor atoms and ring substitutents; (d) the counter anion; and (e) the solvent. The following studies will be carried out during the year. (1) Complete the structural study of the Hg 2 ion dithia-18-crown-6 system. (2) Continue the calorimetric and structural study of metal ion interactions with cyclic polyether-ester molecules looking specifically at possible correlations between the thermodynamic and structural studies. (3) Continue the calorimetric investigation of the interaction of selected metal ions and thia-crown esters.